During the fabrication of many MEMs devices it is required to etch a layer of material to completion stopping on the layer below (e.g., Silicon on Insulator (SOI)—clearing a silicon (Si) layer stopping on an underlying silicon dioxide layer). Allowing the etch process to proceed beyond the time when the first layer has been removed can result in reduced thickness of the underlying stop layer, or feature profile degradation (known in the art as “notching” for SOI applications).
One method commonly used to detect plasma process termination times is optical emission spectrometry (OES). OES analyzes the light emitted from a plasma source to draw inferences about the chemical and physical state of the plasma process. In semiconductor processing this technique is commonly used to detect material interfaces during plasma etch processes. Selwyn's monograph Optical Diagnostic Techniques for Plasma Processing provides an excellent review on the principles and application of plasma emission spectrometry.
The OES technique involves monitoring the radiation emitted by the plasma, usually in the UV/VIS (200 nm–1100 nm) portion of the spectrum. FIG. 1 shows a schematic of a typical OES configuration. The composition of the plasma, and in particular the presence of reactive etch species or etch by-products, is determined by the spectra (i.e., intensity vs. wavelength) of the emitted radiation. During the course of an etch process, and especially at a material transition, the composition of the plasma changes, resulting in a change in the emission spectrum. By continuously monitoring the plasma emission, it is possible for an OES endpoint system to detect a change in the emission spectrum and use it to determine when the film has completely cleared. In practice, most of the information relating to endpoint is usually contained within a few wavelengths that correspond to reactants consumed or etch by-products generated during the etch.
A common method to develop an OES endpoint strategy is to collect a number of spectra of the plasma emission (emission intensity v. wavelength) during both pre- and post-endpoint conditions. These spectra can be used to identify candidate wavelength regions for the process. Candidate regions contain the wavelengths that show a significant intensity change as the process reaches the transition between the two materials of interest. Endpoint wavelength candidate regions can be determined using a number of methods. Spectral regions for endpoint detection can be chosen through statistical methods such as factor analysis or principal component analysis (see U.S. Pat. No. 5,658,423 Angell et al). Another strategy to determine endpoint candidates is through the construction of a difference plot between pre-(main etch) and post-endpoint (over etch) spectra. Once candidate regions have been selected, assignments of likely chemical species may be made for the candidate regions (i.e., reactant species from dissociated gas precursors or etch products). A number of references including Tables of Spectral Lines by Zaidel et al. and The Identification of Molecular Spectra by Pearse et al. in conjunction with knowledge of the process chemistry can be used to assign likely species identities for the candidate lines. An example of likely endpoint candidates for a silicon (Si) etch process in a sulfur hexafluoride (SF6) plasma would be fluorine lines (F) at 687 nm and 703 nm as well as the silicon fluoride (SiF) emission band at 440 nm. Once these regions have been determined, subsequent parts can be processed using the same OES strategy.
While these OES approaches work well for single step processes or processes with a limited number of discrete etch steps (such as an etch initiation followed by a main etch) it is difficult to apply OES to plasma processes with rapid and periodic plasma perturbations. Examples of such processes are the time division multiplexed processes disclosed by Okudaira et al. in U.S. Pat. No. 4,985,114 and Laermer et al. in U.S. Pat. No. 5,501,893. These authors disclose a TDM process for etching high aspect ratio features into Si using an alternating series of etch and deposition steps.
FIGS. 2(a–e) show a schematic representation of the TDM etch process. The TDM etch process is typically carried out in a reactor configured with a high-density plasma source, typically an Inductively Coupled Plasma (ICP), in conjunction with a radio frequency (RF) biased substrate electrode. The most common process gases used in the TDM etch process for Si are sulfur hexafluoride (SF6) and octofluorocyclobutane (C4F8). SF6 is typically used as the etch gas and C4F8 as the deposition gas. During the etch step, SF6 facilitates spontaneous and isotropic etching of silicon (FIG. 2(b)); in the deposition step, C4F8 facilitates protective polymer deposition onto the sidewalls as well as the bottom of etched structures (FIG. 2(c)). The TDM process cyclically alternates between etch and deposition process steps enabling high aspect ratio structures to be defined into a masked silicon substrate. Upon energetic and directional ion bombardment, which is present in etch steps, the polymer film coated in the bottom of etched structures from the previous deposition step will be removed to expose the surface of the silicon for further etching (FIG. 2(d)). The polymer film on the sidewall will remain because it is not subjected to direct ion bombardment, inhibiting lateral etching. Using the TDM approach allows high aspect ratio features to be defined into silicon substrates at high Si etch rates. FIG. 2(e) shows a scanning electron microscope (SEM) image of a cross section of a silicon structure etched using a TDM process.
The plasma emission spectra of etch and deposition steps are dramatically different due to the different plasma conditions that exist in the deposition and etch steps (process gas types, pressures, RF powers, etc.—see FIG. 3). Applying conventional OES methods to a TDM silicon etch process results in an end point trace that is periodic (FIG. 4). It is expected that the majority of the etch endpoint information is contained within the etch segments of the process.
Becker et al. (U.S. Pat. No. 6,200,822) show a method to extract endpoint information from the plasma emission of a TDM process. Becker examines the emission intensity of at least one species (typically fluorine or SiF for a Si etch) in the plasma only during the etch step through the use of an externally supplied trigger (typically the transition from one process step to the next). By using a trigger in conjunction with a sample-and-hold circuit, the emission intensity observed in subsequent etch steps can be stitched together to obtain an emission signal that is not periodic in nature. The value of the emission intensity for the species in the etch step is held at the last known value during the ensuing deposition step. In this manner the periodic emission signal is converted into a curve similar to a step function that can be used for process endpoint determination. The limitations of this approach are the need for an externally supplied trigger in addition to the need for a user input delay between the trigger and acquiring the emission data during etch steps.
In an effort to increase the OES method sensitivity Jerde et al. (U.S. Pat. No. 4,491,499) disclose measuring a narrow band of the emission spectrum while simultaneously measuring the intensity of a wider background band centered about the narrow band. In this manner the background signal can be subtracted from the endpoint signal resulting in a more accurate value of the narrow band signal.
A number of groups have looked at frequency components of plasma emission spectra. Buck et al. (U.S. Pat. No. 6,104,487) describes using digital signal processing techniques such as Fast Fourier Transforms (FFT) to extract frequency components from the plasma emission spectrum. Buck teaches that these components give information about changes in the plasma condition and can be used to detect transitions in the substrate material allowing etch endpoint detection. Buck teaches monitoring infrasonic frequencies down to 10 Hz. Buck considers that the monitored frequencies will change for different processes; however, he only considers steady state (single step) processes, and using FFT optical emission in conjunction with time division multiplexed (TDM) processes that are periodic and repeating is not taught.
Kornblit et al. (U.S. Pat. No. 6,021,215) also describes the use of Fourier transforms in conjunction with optical emission spectroscopy. Kornblit teaches monitoring all frequency components simultaneously, but does not teach the use of optical emission FFT for a TDM process.
Davidow et al. (U.S. Pat. No. 6,455,437) also describes generating frequency components from plasma emissions and monitoring the amplitude of that signal over time. While Davidow contemplates multi-step processes for etching multilayer stacks, the use of optical emission in conjunction with FFT is not taught for TDM processes. Furthermore, Davidow characterizes the plasma process by noting which frequencies are emerging during the course of the process—not examining the magnitude of the characteristic frequencies imposed on the process by the cyclical nature of the TDM process.
O'Neill et al. (U.S. Pat. No. 5,308,414) describes an optical emission system using signal demodulation. O'Neill monitors both a narrow spectral region associated with an etch product along with a wider spectral band to be used as a background correction. O'Neill also discloses frequency demodulation of the signals through the use of a lock-in amplifier. The lock-in amplifier requires an external synchronization signal. O'Neill does not consider multi-step or TDM processes.
Sawin et al. (U.S. Pat. No. 5,450,205) describes a system using optical emission interferometry (OEI) in conjunction with FFT to determine process endpoint. In contrast to optical emission spectrometry (OES) which analyzes the plasma emission, emission interferometry images plasma emission reflected from the wafer surface to determine on-wafer film thickness. Unlike OES, OEI techniques require the imaging detector have a clear line of sight to the wafer surface. The OEI technique is unsuitable for TDM processes that contain repetitive deposition and etch steps due to the cyclical addition and removal of a passivation film as the etch proceeds.
Accordingly, there is a need for an endpoint point strategy for TDM plasma processes that does not require an external trigger to synchronize the plasma emission data collection with the process steps.